The present invention relates to a method and to an arrangement for the rotary drilling of a deviated or high-angle wellbore into the earth's crust which will mitigate the differential sticking of a drill string used to advance a drill bit at the lower end thereof, and which will improve the removal of cuttings settled in the annulus formed about the drill string in the wellbore.
A problem which is frequently encountered in the drilling of high-angle or ultrahigh angle boreholes, in effect, deviated wellbores, consists of in that the cuttings formed by the drill bit at the lower end of the drill string tend to settle on the low side of the borehole. This is particularly the case when the flow of drilling fluid or mud is stopped through the drill string while adding a joint of pipe. The cuttings become difficult to pick up by the return flow of the drilling fluid in the annulus surrounding the drill string when the flow again commences. This problem is seriously compounded because of the flow patterns of the fluid about the drill string. Thus, in a high-angle borehole, the drill string lies on the low side of the hole, and cuttings also tend to settle out on the low side, thereby causing the drill string to rotate in a so called "bed" of cuttings, seriously increasing the tendency for the drill pipe to become "differentially well stuck" or to seize in the wellbore.
Furthermore, drilling fluid velocity in the small areas on the low side of the borehole between the drill string and the hole wall is lower than in the remainder of the borehole, so that cuttings are not readily picked up and conveyed along by the return flow in the borehole.
Recent developments have enabled the drilling and completing of ultrahigh angle wells. Techniques for drilling ultrahigh angles are sometimes referred to as "extended reach drilling", a term that has been coined to describe rotary drilling operations used to drill wellbores at angles which are greater than 60.degree. from the vertical and wherein complex wellbore profiles may be used to extend the horizontal limits of wellbores. Such techniques may be used to provide a wellbore that extends from a surface location to a subsurface location spaced at a considerable lateral distance therefrom.
Among the problems encountered in drilling deviated wells is that of the differential sticking of drill pipe. This problem also is encountered in substantially vertical wellbores but is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe, particularly when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole.